You must update your game from an account with write access to the game folder. You should try updating from the computer's admin account.
You can also manually replace the update program with the latest located here. Save http://files.novalogic.com/DFX2/update.exe to c:\program files\NovaLogic\Delta Force Xtreme 2\
If you wish to do a full manual update of the game, download the latest installer and "repair" your install to the latest version. The latest full installer is located here (note: the download can take awhile)
After downloading, just run the program "repair" the installation. This will bring you to the latest version